Language Design: Equality & Identity

Part 6: Fixing Rust

Rust designers recognized the issues with Haskell’s approach, but were not able to address the issues with Rust’s Eq and PartialEq traits.

The main cause of this failure is the sub-typing relationship between PartialEq and Eq:

It requires that an implementation of partial order is consistent with an implementation of total order.

This works for many types, but not for floating point types, for which the IEEE754 standard specifies a partial order as well as a total order.

The easiest solution for Rust would have been to not introduce the sub-typing relationship between PartialEq and Eq (as well as between PartialOrd and Ord).